The invention relates to a method for monitoring the vehicle handling of a vehicle, in particular a two track vehicle, with respect to the vehicle stability by comparing a measured variable, which reflects the yaw rate or the lateral acceleration of the vehicle, with a comparison value, which is derived from the steering angle, specified by the driver of the vehicle.
Vehicle dynamic control systems, which stabilize, for example, a passenger vehicle in critical driving situations, are known in principle, for example, in the form of the so-called Electronic Stability Program (ESP). Such systems produce, for example, a yaw moment, which is directed against an oversteering of the vehicle. However, at the same time it is necessary to detect as early as possible an oversteering or understeering of the vehicle with adequate accuracy. The more accurate this detection, the more effectively a stabilization function (of any kind whatsoever) can be applied.
One possible method for detecting the understeering or oversteering is to compare the measured vehicle dynamics variables, such as the yaw rate and/or the lateral acceleration, with corresponding simulated values derived from a linear vehicle model. In this case, the difference between the measurement and the simulation is interpreted as a measure of the nonlinear characteristics of a current vehicle reaction. However, this method has the drawback that the model that is used has to match, exactly as possible, the individual vehicle parameters, like mass, inertia moment, sideslip stiffness of the tires, etc. A variation in these parameters, for example due to loading, upsets the model and, thus, increases the deviation between the measurement and the stimulation so that, when this model is used, it is quite possible that the current driving condition may be interpreted as nonlinear even though the vehicle exhibits a linear and, thus, non-critical characteristic.
The present invention provides an improved method for monitoring the vehicle handling of a vehicle, in particular a two track vehicle, with respect to the vehicle stability by comparing a measured variable, which reflects the yaw rate or the lateral acceleration of the vehicle, with a comparison value, which is derived from the steering angle, specified by the driver of the vehicle.
According to the invention, the gradients of the regression lines, which are normalized in relation to each other, of the measured variable, on the one hand, and the comparison value, which represents the steering angle, on the other hand, are compared continuously over a number of analysis time windows. Upon finding a significant deviation between these gradients, an unstable driving condition is concluded.
It was recognized that a nonlinear and, thus, unstable vehicle handling of a vehicle can be determined by finding the gradient differences by use of a regression analysis without having to take into consideration the individual vehicle parameters, such as the vehicle mass or the actual type of tire and the like. The invention analyzes, in particular, only whether there is a significant difference between the respective gradient(s) between the temporal course of an input value of the system to be examined—thus, in this case between the temporal course of the driver's specified steering angle and a corresponding system response—that is, the temporal course of a system output value in the form of the lateral acceleration and/or yaw rate of the vehicle. If, therefore, there is a significant difference, then it is possible to conclude an unstable driving condition. For example, at the beginning of a typical stationary understeering, the gradient of the steeling angle remains unchanged over time, whereas the gradient of the lateral acceleration and the yaw rate decreases, as viewed over time.
Preferably, the difference between the gradients of the regression lines, which are determined by a first order least squares method and normalized to the same units by a suitably applied amplification factor, is determined and compared with a suitably determined limit value, above which an instability is inferred. Thus, even if the boundary conditions change, such as loading and, thus, in the event of a varying mass of the vehicle, or in the event of a change in the inertia moment, or in the event of a varying sideslip stiffness of the tires, and the like, it is possible with this method to monitor with high robustness the vehicle handling with respect to the vehicle stability in a simple manner and with a minimum application complexity over the entire service life of the vehicle. If a gradient difference, which is referred to as significant, is detected, then at the current time there exists an unstable driving condition, and a suitable chassis control system may intervene in a stabilizing manner.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description when considered in conjunction with the accompanying drawings.